Security has become an increasing concern in both the home and office with the acquisition of expensive appliances such as computer, printers, or high fidelity stereo systems. These types of appliances are particularly attractive for theft because they are easily transported and fenced but difficult to trace. The danger of theft may be greater in the home than in the office because the value of appliances rarely justifies the cost of an expensive security system. This is especially true for those that rent and cannot recover the cost of a security system.
Realizing this need, particularly in the lodging industry, prior art devices have been designed that monitor a circuit connection between the appliance and a power source such as a wall socket or antenna. If the appliance is unplugged, these devices detect the broken circuit and sound an alarm. However, these prior devices have suffered from a number of drawbacks that have discouraged their widespread use, including questionable reliability, cost, and ease of use. For example, U.S. Pat. No. 3,407,400 to Lurie discloses a theft control system that requires modification of an existing appliance by the addition of a resistor to its power cord. The resistor is added to construct one arm of an impedance bridge formed through the ground prong of the appliance plug. Not only is this device difficult to install for the homeowner, it raises questions of safety since the ground prong is no longer connected to ground. Moreover, nothing in the device prevents the disconnection of the device itself from the power source to circumvent the alarm.
U.S. Pat. No. 3,794,989 to Manley et al. gives an example of another approach in the prior art. Manley establishes a shunt circuit through the ground and neutral prongs of a plug in parallel with an alarm circuit. If the shunt circuit is broken, current flows instead through the alarm circuit to sound an alarm. However, the shunt circuit of Manley is established through rewiring of the ground and neutral plugs of the device to establish the shunt to ground. Not only is this rewiring a difficult task for the homeowner, it also raises questions of a reliable ground from the chassis of the appliance.
Yet a different approach is disclosed in U.S. Pat. No. 4,040,045 to Osborne et al. Osborne utilizes the antenna connection to a television set as a means for detecting disconnection of the set. The resistor is connected across the lines of the antenna connection. If the television set is disconnected, a change in resistance of the antenna line is detected and an alarm sounds. This device, as the other devices of the prior art, requires a complicated installation of elements in an antenna system. Furthermore, the Osborne device works only with appliances that require antennas and have a coaxial connection. The Osborne device is not suitable for appliances such as computers, food processors, or stereo systems that lack coaxial cable-type antennas.
Other examples of prior art devices that have the same or similar drawbacks are disclosed in U.S. Pat. No. 3,644,912 to Allen, Jr.; U.S. Pat. No. 4,005,399 to Pazlmenas; U.S. Pat. No. 4,032,916 to Galvin et al.; and U.S. Pat. No. 4,300,130 to Fotheringham.